This invention relates to a process for the preparation of nitromethylene heterocyclic compounds.
UK Patent Specification No. 1,513,951 discloses a group of pesticidally, particularly insecticidally and acaricidally, active nitromethylene derivatives of oxazine and thiazine. UK Patent Specification No. 1,576,120 discloses the utility in combating insect pests of rice crops of a class of nitromethylene heterocyclic compounds of formula ##STR2## wherein n is 2 or 3, X inter alia represents S, R.sub.1 inter alia represents a hydrogen atom and R.sub.2 inter alia represents a hydrogen atom, 2-nitromethylenetetrahydro-2H-1,3-thiazine being specifically exemplified.
U.S. Pat. No. 4,052,388 discloses 3-acetyl-2-nitro-methylene-tetrahydro-2H-1,3-thiazine and its insecticidal activity. EP-A-117577 discloses 3-formyl-2-nitromethylene-tetrahydro-2H-1,3-thiazine and its pesticidal, particularly insecticidal activity. The preparation of the 3-acetyl- and 3-formyl-derivatives is via 2-nitromethylene-tetrahydro-2H-1,3-thiazine.
According to UK Patent Specification No. 1,513,951, 2-nitromethylene-tetrahydro-2H-1,3-thiazine may be prepared by either of two methods:
Method A: treating nitroketene dimethyl mercaptole (R. Gompper & H. Schaefer, Chemische Berichte, 100, 591 (1967)) with 3-amino-1-propanethiol (S. D. Turk, et al., J. Org. Chem., 27, 2846 (1962)); and
Method B: treating 5,6-dihydro-2-(methylthio)-4H-1,3-thiazine (A. F. McKay et al., J. Am. Chem. Soc., 80,3339 (1958)) with an alkyl, e.g., ethyl, nitroacetate (S. Zen et al., Kogyo Kagaku Zasshi, 74, 70 (1971)) in the presence of a catalytic amount of zinc ion (e.g., zinc chloride) to form the alkyl nitro (tetrahydro-2H-1,3-thiazine-2-yl-idene) acetate, which is hydrolyzed with a base and decarboxylated by acidification to give the desired product.
In EP-A-127413 it is indicated that Method A has the disadvantage that 3-amino-1-propanethiol is not readily accessible, being an expensive material, and overall yield is only moderate, and that Method B suffers from the disadvantage that ethyl nitroacetate is not available in commercial-scale quantities.
EP-A-127413 itself discloses a process for the preparation of compounds of formula ##STR3## where each R is hydrogen or an appropriate aromatic or aliphatic substituent, the method comprising reacting together a sulphur donor, 1,1-bis(methylthio)-2-nitroethene and a compound of the formula EQU H.sub.2 NCR.sub.2 CR.sub.2 CR.sub.2 OSO.sub.3 H
where each R is as defined above. Preferably, each R is independently hydrogen or C.sub.1-4 alkyl and more preferably each R is hydrogen. The sulphur donor may be any suitable source of sulphur, for example, sulphur itself, a sulphide, a hydrosulphide or hydrogen sulphide. Preferably, the sulphur donor is an ammonium or alkali metal sulphide or hydrosulphide, e.g., sodium sulphide.
In the process of EP-A-127413, two mols of methanethiol are produced per mol of desired end product. There has now surprisingly been developed an improved process wherein in the preparation of such products the stoichiometry requires only one mol of methanethiol to be produced per mol of desired end product.